The invention relates generally to superconducting magnets and, particularly, to quench protection of superconducting magnets.
In general, superconductors are composite materials in the form of wires or flat tapes (e.g., round or rectangular). The composite conductor typically includes copper or silver for protection and stabilization in addition to a superconducting alloy or compound. The composite conductor may also have substantial fractions of other materials (e.g., bronze). Known high temperature superconductors (HTS) operating at typical temperatures of less than about 80 K include, among others, BSCCO, YBCO, and MgB2. Known low temperature superconductors (LTS) operating at typical temperatures of less than about 10 K include Nb3Sn and NbTi. A superconducting magnet often employs superconductors in a set of epoxy impregnated long solenoids.
Because superconducting magnets are often designed to produce high magnetic fields, they store relatively large amounts of magnetic energy during operation. Unfortunately, this stored energy may subject such a magnet to a mode of failure, known as “quench,” in which the stored energy is suddenly converted into heat accompanied by the presence of large electrical voltages. A quench usually occurs when a conductor transitions from the superconducting state to the normal state in some region of one of the magnet coils. In the normal, non-superconducting state, the conductor has an increasingly large electrical resistance relative to the superconducting state and is heated by the current in the magnet.
A local dissipation of energy, for example, causes dissipation of the magnetic energy stored in the windings and leads to discharge of the magnet. Normal zone propagation, which is largely governed by the rate of thermal diffusion in solenoid wound coils, increases the size of the initial normal zone and dissipation of energy in this zone increases the temperature in the magnet. The temperature increase is governed by the resistivity and heat capacity of the windings. Also, the maximum temperature resulting from the quench depends on the initial current density and the discharge time.
The excess temperature and voltage in the windings caused by a quench condition can potentially damage the magnet. For example, when the stored magnetic energy is deposited over a limited volume of the magnet, the energy density and temperature can be high. A local resistance associated with a limited volume, and the increase in resistivity with temperature, can result in a localization of the resistive voltage in the windings of a magnet that is not balanced by the distributed inductive voltage during the discharge of the magnet. This leaves a relatively large physical voltage in the windings.
Although systems are known for protecting a superconducting magnet from damage due to a quench fault condition, improvements are desired. Typical magnet protection schemes aim to eliminate the potential adverse effects of high temperature and voltage during quench. Often, protection is achieved both through characteristics of the conductor in the magnet (e.g., by the addition of low resistivity stabilizer in a composite superconductor) and through characteristics of the overall circuit associated with the magnet (e.g., by extending normal zone volume in the magnet and rapidly discharging the stored energy).
With some superconducting magnets, it is possible to remove the stored energy from the coil using an external dump resistor and switch. When a quench detector senses a quench condition in the magnet, a protective circuit opens the switch to essentially create a series circuit of inductor and resistor. The magnet largely deposits its stored energy in the external resistor as it decays with a time constant characteristic of such circuits.
One alternative to removing the magnetic stored energy during a quench condition is to dissipate the energy internally to the magnet windings. A quench is usually a local phenomenon and, thus, the energy will dissipate locally. In this instance, the local region will overheat and be damaged if enough energy is available in the magnet. Distributing the energy somewhat uniformly over the entire volume of the magnet will help prevent overheating any one portion of the windings. Conventional protection systems are available for distributing the stored energy in the magnet. The particular type of system used depends on the type of magnet involved.
Conventional quench protection systems are usually classified as active or passive, and as external or internal. In addition, these protection systems are classified by whether they do or do not contain protection heaters. A simple active protection circuit consists of a room temperature circuit breaker switch in series with the magnet power supply, and an external dump resistor, as shown in FIG. 1. Such an active system also requires a quench detector to recognize the presence of a quench condition and to open the switch. FIG. 2 illustrates a simple passive protection circuit, which consists of a subdivision of the magnet windings into distinct coils accessible to a shunt loop containing a resistor or a combination of a resistor and a series diode. Such a passive system generally responds to the conditions that result from a quench, namely, the rise in resistance locally in a coil, to prevent excessive temperature and voltage in the windings.
Conventional quench protection systems are sometimes more complicated. For example, in an active system such as the one illustrated in FIG. 3A or FIG. 3B, a quench detector activates an external heater power supply to energize a heater that is in thermal contact with the coil windings. This system is commonly used on LTS accelerator magnets with limited numbers and distribution of heaters. In a variation of this system, as shown in FIGS. 4A and 4B, the external heater power supply activates a heater in a secondary persistent switch included in the magnet circuit, driving the switch normal and resistive, and forcing a portion of the magnet current to flow in a parallel circuit of resistive heaters that are in thermal contact with the magnet. The active protection system of FIGS. 4A and 4B, is commonly used on LTS high field NMR magnets with heaters placed on the outside surface of the windings of each coil.
Similar arrangements can be configured as a passive systems, as shown in FIG. 5A and FIG. 5B, by using the voltage that develops across the coil, or preferably across the coil section containing the initial normal zone, to power the heater in the persistent switch. When the switch goes normal, the magnet current flows in the parallel heaters located on the outside surface of the windings.
Unfortunately, the existing quench protection systems are generally limited to LTS magnets. As such, the protection heaters used in the prior art that are in good thermal contact with the coil windings are designed to be consistent with the behavior and protection requirements of LTS magnets. Because the normal zone propagation rate is typically rapid in LTS coils, the protection heaters need only be few in number and are typically placed on the outside surface of coil windings. The normal zone created by such heaters spreads rapidly in low temperature superconductors to encompass an extended volume of the windings and thereby serves the purposes of quench protection. But such conventional protection systems are inadequate for use in HTS magnets, which have a much slower normal zone propagation rate.
High temperature superconductors are characterized by high critical temperatures and high critical fields. The high critical temperature allows for operation of HTS coils at relatively high temperatures compared to LTS coils. For example, coils containing YBCO superconductor are expected to operate in a range of cryogenic nitrogen, 65 K to 77 K. At these temperatures, the heat capacity of materials is very large compared to the heat capacity near liquid helium temperatures (e.g., 4 K). In addition, when HTS conductors such as those containing BSCCO or MgB2 are used at liquid helium temperatures, the high critical field allows for operation of magnets at values of field exceeding those possible with LTS conductors, while still having a critical temperature far greater than that of LTS conductors. High heat capacity and large temperature margins below critical temperature result in a high stability of the magnet against thermal disturbances because of the large energy necessary to create an initial normal zone. Although less likely than in an LTS coil, quench is still a possibility in an HTS coil. Sources of excess local temperature to initiate quench may arise from a failure of the cryogenic system to maintain the low temperature and from ac loss heating of the windings. Furthermore, when an HTS coil is used in a magnet containing additional LTS coils, the HTS coil must be protected against the effects of a quench in any coil in the magnet. Given the potential for a quench, quench protection of the magnet is required to assure equipment safety.
The increased stability of HTS coils makes quench protection more difficult once a quench does occur. The relatively high heat capacity and temperature margin of HTS coils compared to LTS coils result in a low rate of normal zone propagation. The lack of rapid normal zone propagation is a dominant factor in the design of protection of HTS coils.
Some currently available quench protection systems, such as active protection with an external dump resistor (see FIG. 1), do not depend on the rate of normal zone propagation. The operation of a typical active protection system with external energy dump requires a discharge voltage, for a given decay time constant, that is proportional to the stored energy of the magnet and inversely proportional to the operating current of the magnet. Typically, external energy dumps are used with large magnets of high stored energy and high operating current. At low operating current, the required discharge voltage is excessively high for the dielectric design of such magnets. Even with reduced stored energy in relatively small coils, the discharge voltage can be significant (e.g., on the order of 1000 volts). The discharge voltage is common to the dump resistor, the switch that opens the circuit, and the magnet, as shown in FIG. 1. The high switch voltage results in complexity, size, and weight in the switch. High voltage discharge has practical limitations and also places constraints on the dielectric design of the magnet coils. Those skilled in the art are familiar with the use of an external energy discharge at high voltage in an attempt to protect an HTS insert coil operating in a test mode in the large bore of an outer test coil. Nevertheless, a conventional external dump resistor protection system is not well suited for the protection of coils operating at relatively low current, as is typical with HTS coils.
Passive protection systems that internally deposit stored magnetic energy are common for LTS coils with relatively low current and low stored energy, and are increasingly being applied to LTS coils of low current and higher stored energy. Passive protection systems, such as shown in FIG. 2, generally rely on the rapid growth of normal zone resistance for transferring inductive energy among the coils. In this instance, the inductive energy transfer results from a growth in the resistance of a coil that is well coupled by mutual inductance to neighboring coils. Those skilled in the art are familiar with a stand alone developmental HTS coil for a gyrotron application that used a passive circuit with shunts. For the gyrotron application, the resistive shunts were external to the windings of the coil and high thermal conductivity sheets of copper were distributed in the windings and thermally connected to the shunts as a way to distribute the heat deposited in the shunts throughout the coil. The lack of fast resistive growth in HTS coils makes such systems ineffective, as well as any other system that depends for its operation on the rapid increase of resistance and voltage. Therefore, passive protection systems provide inadequate quench protection for HTS coils.
Although active protection systems that dissipate energy internally have been used on LTS coils, the heater configurations associated with such systems are limited to a relatively small number of heaters placed largely on the outside surface of the windings. Unfortunately, distributing heaters in this manner fails to effectively protect of HTS coils from quench conditions because of the lack of normal zone propagation from the surface of the windings through the volume of the coils.
Commonly assigned U.S. Pat. No. 6,735,848, the entire disclosure of which is incorporated herein by reference, discloses a superconducting magnet employing an active quench protection circuit.
In light of the foregoing, improvements are desired in the protection of superconducting magnets, especially those containing HTS coils.